Nearly all commercially made VCM is produced by thermal dehydrochlorination or cracking of EDC.
C2H4Cl2 → C2H3Cl + HCl
Cracking is an endothermic process (ΔHrxn = 71 kJ/mol EDC consumed) that occurs as a homogeneous, vapor-phase, first-order, free-radical chain reaction. Commercial EDC crackers operate at gauge pressures of 1.4-3.0 MPa (200-435 psig) and at temperatures of 475-525°C. EDC conversion per pass is normally maintained at 53-63%, with a residence time of 2-30 sec. Cracking reaction selectivity to vinyl chloride of >99% can be achieved at these conditions. Increasing conversion beyond this level gives progressively lower selectivity to VCM and higher coking rates.
Cracking furnace effluent must be quenched, or cooled rapidly, to keep coking at a minimum. Therefore, the hot effluent gases are typically quenched and partially condensed by direct contact with cold EDC in a quench tower. Alternatively, the hot effluent can first be cooled by heat exchange with cold liquid EDC furnace feed or by vaporizing boiler feed water (BFW) to produce high pressure steam in a transfer line exchanger (TLX) prior to entering the quench tower. This arrangement saves energy by decreasing the amount of fuel needed to fire the cracking furnace and/or steam needed to vaporize the feed.
EDC Cracking and VCM Purification Process
